1. Field of the Invention
The present invention relates to a light-emitting device. In particular, the present invention relates to a light-emitting device suitable for an image-writing device to be used in an optical printer such as an optical exposure type printer employing a self-developing type instant film as a recording paper. The present invention further relates to a recording device using the above-mentioned device.
2. Description of the Related Art
In recent years, along with the spread of electronic still cameras, digital video cameras, and the like, demand for a printer or a video printer, which requires no chemical treatment for development or the like and is capable of instantaneously outputting a color photography print based on a digital signal obtained from the above-described devices, has been rapidly increasing in the market targeted for personal use.
In such a video printer, there are presently two writing methods for printing paper, i.e., a sublimation type heat-transfer method and an optical exposure method. Although the sublimation type heat-transfer method is presently employed more often, a printer employing this type of writing method has disadvantages. For example, there is a limitation to the miniaturization of the printer set due to the need for an ink ribbon. In addition, running cost per recording paper is high when the cost of the ink ribbon is taken into consideration. Moreover, in the printer employing the sublimation type heat-transfer method, an image writing unit thereof has a line-shaped writing structure, the entire cost of the printer also becomes high.
On the other hand, in the optical exposure method, a recording paper is photosensitized by light irradiation to be developed. Since self-developing type recording paper is used in this method, no ink ribbon is required, realizing miniaturization of the printer set. However, most-frequently used recording paper at present is a silver-salt type instant film, resulting in high running cost per recording paper as in the sublimation type printer. Additionally, upon producing the recording paper, chemical treatment using silver salt or the like is required.
In recent years, a recording paper which is photosensitized with the respective wavelengths of the three primary colors, i.e., red (hereinafter, simply referred to as "R"), green (hereinafter, simply referred to as "G"), and blue (hereinafter, simply referred to as "B") has been developed, and has attracted great attention. Specifically, on such a recording paper, there exist photosensitive micro-capsules each having a property to react to one of the light wavelengths of R, G, and B. The recording paper is developed by exposing the micro-capsules to light. In this method, the cost per recording paper is reduced to about 1/3 of that in the silver-salt method.
With respect to light-emitting diodes (hereinafter, referred to as an "LEDs"), LEDs which emit light in the blue wavelength region or in the green wavelength region ranging from blue to green at a high luminance have been developed recently, and thus, the three primary colors of R, G, and B can be obtained at a high luminance. The LEDs attracts greatest attention as an exposure beam source for exposing the above-described micro-capsules, and are expected to be widely used in future.
In the developing method where the micro-capsules are exposed to light, an LED head is used in a printing head unit as a light-emitting device used for an image writing device, and specifically, an LED lens optical exposure method is presently suggested as the structure thereof.
Hereinafter, the conventional image writing LED head will be described with reference to FIG. 27. FIG. 27 is an enlarged cross-sectional side view illustrating a main portion of a printing head unit employing such a conventional LED head.
The printing head unit basically includes: a printed circuit board 1; LED elements 4 (only one element is shown for simplicity); a plate 2 having an aperture 9;
and a spherical lens 8. In accordance with printing information supplied from a controlling unit (not shown), each of the LED elements 4 arranged as a set respectively corresponding to the three primary colors of R, G, and B emits light, and the light energy condensed by the spherical lens 8 is sequentially irradiated onto a recording paper 3 in the form of a flying spot. In this manner, the recording paper 3 is photosensitized.
Next, the above structure and its operation will be described in more detail.
One electrode in each of the LED elements 4 arranged as the set of R, G, and B is attached to a predetermined conductor pattern (not shown) on the printed circuit board 1 with conductive adhesive. The other electrodes of the respective LED elements 4 are connected to the different conductor patterns on the printed circuit board 1 with metal thin wires 5, respectively. Moreover, the aperture 9 and the spherical lens 8 are disposed directly above the LED elements 4.
The operation of the thus structured LED head is as follows.
First, a digital image is divided into image data each corresponding to one pixel (hereinafter, referred to as the pixel image data), and the pixel image data is converted into a color map. Then, the color map is converted into digital signals each of which is assigned for each pixel. The LED elements 4 emit light in accordance with the transmitted digital signals so as to be focused upon a pixel on the recording paper 3 via the aperture 9 and the spherical lens 8 located directly above each of the LED elements 4. Radiation power on the recording paper 3 causes the micro-capsules thereon to be exposed and photosensitized. The writing of an image is performed for all of the pixels on the recording paper 3 by sequentially exposing and photosensitizing the recording paper 3 in the above manner.
However, the conventional device as described above still suffers from disadvantages.
In the market, there is a demand for a reduction in printing time (i.e., time required for recording and exposing) and a light-weight miniaturized printer set as improvement in the functions of a printer. In order to achieve a reduction in printing time, in particular, there exists a need for improving an utilization efficiency of the entire radiant flux from the LED elements and thus increasing radiation power on the recording paper.
In this regard, in the above-described conventional LED head, only a part of radiant flux passed through an opening of the spherical lens 8 via the aperture 9 is condensed among the entire radiant flux emitted from the LED elements 4. Thus, as compared to the entire radiant flux emitted from the LED elements 4, actual radiation power on the recording paper 3 is low.
In order to increase the radiation power on the recording paper 3, it is most effective to enlarge the opening of the spherical lens 8, i.e., to increase a diameter of the lens 8. However, the increased lens diameter results in a longer optical path length and disadvantages associated with an aberration, thereby preventing an uniform and sharp light beam from forming.
Thus, in the conventional LED head, it is difficult to improve the utilization efficiency of light above the level currently obtainable. As a result, it is difficult to increase radiation power on the recording paper, and thus the printing time cannot be shortened.
Moreover, in the conventional optical system, the optical path length from the LED elements 4 to the recording paper 3 is long, and therefore, its miniaturization is hard to be realized. In addition, high assembling accuracy and large number of steps are required for adjusting the focusing condition, focus control is hard to be performed.
Furthermore, there are many problems including those described hereinafter. For example, since a beam diameter varies due to color aberration resulting from differences among wavelengths, the resolution is thereby adversely affected. In addition, since a beam diameter cannot be made smaller than the luminescent size of the LED elements, resolution smaller than the luminescent size of the LED elements is hard to be realized.
Moreover, some LED elements are susceptible to static electricity due to the physical constant (e.g., dielectric constant c) of a constituent material or a structure of the LED element.